Mechanism of the anti-proliferative action of 25-hydroxy-19-nor-vitamin D3 in human prostate cells

According to the prevailing paradigm, 1α-hydroxylation of 25-hydroxyvitamin D3 (25(OH)D3) and its analogs is a pre-requisite step for their biological effects. We previously reported that 25-hydroxy-19-nor-vitamin D3 (25(OH)-19-nor-D3) had anti-proliferative activity in a cell line, PZ-HPV-7, which was derived from human non-cancerous prostate tissue, and suggested that 25(OH)-19-nor-D3 acted after 1α-hydroxylation by vitamin D 1α-hydroxylase (CYP27B1). However, metabolic studies of 25(OH)-19-nor-D3 using recombinant CYP27B1 revealed that 25(OH)-19-nor-D3 was rarely subjected to 1α-hydroxylation. Therefore, in this report, we attempted to clarify the mechanism of 25(OH)-19-nor-D3 action in intact cells using PZ-HPV-7 prostate cells. After incubating the cells with 25(OH)-19-nor-D3, eight metabolites of 24-hydroxylase (CYP24A1) were detected, whereas no products of CYP27B1 including 1α,25-dihydroxy-19-nor-vitamin D3 (1α,25(OH)2-19-nor-D3) were found. Furthermore, the time-dependent nuclear translocation of vitamin D receptor (VDR) and the subsequent transactivation of cyp24A1 gene in the presence of 25(OH)-19-nor-D3 were almost identical as those induced by 1α,25(OH)2-19-nor-D3. These results strongly suggest that 25(OH)-19-nor-D3 directly binds to VDR as a ligand and transports VDR into the nucleus to induce transcription of cyp24A1 gene. In addition, knock down of cyp27B1 gene did not affect the anti-proliferative activity of 25(OH)-19-nor-D3, whereas knock down of VDR attenuated the inhibitory effect. Thus, our results clearly demonstrate that the anti-proliferative activity of 25(OH)-19-nor-D3 is VDR dependent but 1α-hydroxylation independent, suggesting that 25(OH)D3 analogs such as 25(OH)-19-nor-D3 could be attractive candidates for anticancer therapy.